X-ray or gamma ray systems or devices – Electronic circuit – X-ray source power supply
Reexamination Certificate
1998-11-25
2001-02-20
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Electronic circuit
X-ray source power supply
Reexamination Certificate
active
06192105
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to x-ray imaging systems using automatic exposure control devices. More particularly, this invention relates to a method and device to assist in calibrating automatic exposure control devices used in x-ray imaging systems.
BACKGROUND OF THE INVENTION
It is known in the art to use an automatic exposure control (AEC) device to control the exposure of x-rays in an x-ray imaging system. An AEC device is generally placed after the subject being imaged and prior to the imaging cassette or detector, although it may also be placed after the cassette or detector. The purpose of the AEC device is to sense a small fraction of the x-rays which have passed through the patient and generate an electrical signal indicative of the x-ray exposure of the imaging cassette or detector. Once the correct x-ray exposure is obtained, as determined from the AEC signal, the exposure is terminated.
It is important in diagnostic x-ray imaging to produce images with a consistent optical density or image quality so that a more accurate diagnosis can be made. Consistent optical density or image quality is also important so that accurate comparisons can be made to previous images.
It is known in the art to record an x-ray image on a film by placing the film in a cassette having at least one phosphor screen, and preferably two phosphor screens, with a double emulsion film sandwiched between the two screens. The screens may be of different thicknesses. The phosphor screens emit light fluorescently in response to x-rays, thereby converting the x-ray image into another medium, namely light. The fluorescent light emitted by the phosphor screen is recorded on the film, thereby recording the x-ray image.
The signal from the AEC device is related to the fluorescent light exposure of the film in the cassette or on the detector in a complicated manner. AEC devices are typically comprised of ion chambers or thin solid-state x-ray detectors. There may be one, two, three or more fields in an AEC device. The response of AEC devices to x-ray radiation differs considerably from that of screen/film systems or digital detectors. The AEC device must have a low quantum efficiency (“QE”) so that a very small fraction of the x-ray radiation is absorbed by the AEC device, intercepting but a very small fraction of the x-ray radiation, since any intercepted radiation does not contribute to the final image and leads to increased patient exposure. Also, by intercepting a small amount of the image radiation, it is less likely that a noticeable image of the AEC detector will appear in the final radiograph.
The low QE requirement of the AEC detector typically results in an AEC detector design which has a response to x-ray radiation which is different than the imaging sensing and recording device. Therefore, the AEC detector response varies differently to changing x-ray conditions than the image sensing device. For example, the x-ray spectrum changes due to a change in x-ray tube voltage (kV) and changes in the patient anatomy and thickness. Hence, it is necessary to accurately calibrate the AEC device to determine a correct and consistent relationship between the x-rays being detected by the AEC device and the desired fluorescent light exposure of the screen/film combination or digital detector. More particularly, the calibration procedure will result in data indicating the desired output signal of the AEC device which corresponds to a desired optical density and image quality or digital signal for the particular conditions, such as generator kV, patient anatomy and/or thickness, screen/film combination and film processor speed.
In the past, AEC devices have been calibrated using a tedious trial and error approach. Because the exposure on a film will depend on several variables, such as the x-ray generator kV, the patient anatomy and/or thickness, the screen/film combination and the film processor speed, several different exposures involving development of several films or digital images is required to properly calibrate the AEC device for each of the variables. In addition, the screen/film combination must be calibrated in each receptor where it may be located, such as in the table or on the wall. An imaging system may have more than one, such as four, receptors. This process can take many hours to complete for each different combination of x-ray generator kV, patient thickness, screen/film combination and film processor speed. Also, the AEC device must be recalibrated each time there is a change in one or more of the variables of the x-ray imaging system, such as a change in the screens or films used, installation of a new x-ray generator, replacement of the x-ray tube, a grid change or a change to the added filtration in the x-ray collimator.
Therefore, while AEC devices are useful in automatically obtaining the proper exposure for films, the prior art method for calibrating the AEC device is tedious, time consuming, and requires exposing and developing several films, on the order of fifty or more. This all increases the cost of installation and calibration and also decreases the amount of time the x-ray imaging system is available for imaging.
In addition to film/screen x-ray imaging systems, there is a move towards digital recording x-ray imaging systems. In digital recording systems, the x-ray image is recorded in a digital or electronic form. One class of digital systems include Cesium Iodide or phosphor screen systems which convert the x-rays to light. These classes of digital systems utilize a variety of image sensing devices, such as (1) direct optical coupling to active matrix thin film transistor (TFT) switching arrays having a photodiode or other light sensing means at each matrix position (flat panels), (2) charge coupled devices (CCDs) or (3) integrated CMOS detector technology devices. Direct optical coupling generally has no magnification factor while charge coupled devices and integrated CMOS detector technology devices record a magnification reduced light image after it has been optically coupled to the sensors via lenses or fibre-optics. A further class of digital systems include photostimuable phosphor systems wherein the x-ray image is captured as a latent image on a storage phosphor plate which can then be readout by a laser scanning device. Other classes of digital systems may utilize x-ray sensitive photoconductors such as amorphous selenium or lead oxide to convert the x-ray image directly into an electric charge which can then be directly sensed, recorded and transferred electronically using TFTs, diode switching arrays, or, readout by laser scanning methods.
As the digital recording systems also utilize AEC devices to control x-ray exposure, the digital recording systems must also be calibrated and optimized to give radiographs that yield the proper image quality and x-ray exposure levels. This process may involve a careful adjustment that relates the response of the AEC device signal to the response of the digital detector which detects the converted medium, whether it is light, an electric charge, or another medium.
Accordingly, there is a need in the art for an improved method and device to automatically and efficiently calibrate AEC devices. Furthermore, there is a need in the art for a method and device to calibrate AEC devices which does not require a large number of exposures, radiation level measurements, and the development of a large number of films or sequencing of digital images.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to at least partially overcome the disadvantages of the prior art. Also, it is an object of this invention to provide an improved type of device and method to automatically calibrate AEC devices. Furthermore, it is an object of the present invention to provide a method and device to more quickly calibrate AEC devices without the need to make a large number of x-ray films, x-ray exposures or radiation level measurements.
Accordingly, in one of its aspects, this invention resides in a device for calibrating a
Hunter David MacKenzie
Joy Michael L.G.
Bruce David V.
Communications & Power Industries Canada Inc.
Hobden Pamela R.
Pervanas Jeffrey
Riches McKenzie & Herbert LLP
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